1. Field of the Invention
The present invention relates to a method for producing a cyclic aliphatic oxime. More particularly, the present invention is concerned with a method for producing a cyclic aliphatic oxime, which comprises subjecting a cyclic aliphatic primary amine to oxidation in the liquid phase under superatmospheric pressure in the presence of molecular oxygen and a solid catalyst, wherein the solid catalyst comprises an oxide of at least one metal (a) selected from the group consisting of metals belonging to Groups 5 and 6 of the Periodic Table. The method of the present invention is advantageous not only in that the oxidizing agent employed is molecular oxygen which is much safer than other oxidizing agents (such as hydrogen peroxide and an organic hydroperoxide), but also in that it becomes possible to produce a cyclic aliphatic oxime in the liquid phase with high selectivity. Further, since a solid catalyst is used in the method of the present invention, the separation and recovery of the catalyst from the reaction mixture can be very easily performed after the reaction without any cumbersome operations.
2. Prior Art
A cyclic aliphatic oxime is a compound which is useful as an antioxidant or the like, but it is also widely used as a raw material of products (such as medicines and agricultural chemicals) related to the field of organic industrial chemistry. A cyclic aliphatic oxime is obtained by an oxidation of a cyclic aliphatic primary amine and, for example, when the cyclic aliphatic primary amine is cyclohexylamine, cyclohexanone oxime is obtained as a corresponding cyclic aliphatic oxime. Cyclohexanone oxime is a commercially important compound which is used as not only an antioxidant, but also an intermediate of ε-caprolactam which is a raw material of nylon-6.
As conventional methods for producing an oxime by oxidizing an alicyclic or aliphatic primary amine, there can be mentioned a method in which an alicyclic or aliphatic primary amine is reacted with hydrogen peroxide in the presence of an inorganic salt catalyst comprising molybdenum, tungsten or uranium as an active metal species; and a method in which an alicyclic or aliphatic primary amine is reacted with an organic hydroperoxide in an organic solvent in the presence of a catalyst comprising titanium, molybdenum, tungsten and vanadium. When these conventional methods which employ hydrogen peroxide or an organic hydroperoxide as an oxidizing agent are practiced on a commercial scale, the oxidizing agent is handled by a conventional manner of operation, which is dangerous due to the very high reactivity of such oxidizing agent. In addition, when an organic hydroperoxide is used, since by-products formed by the reduction of the organic hydroperoxide are contained in the resultant reaction mixture, the separation and purification of the desired oxime become cumbersome.
In order to solve the above-mentioned problems, the following methods (1) to (4) which use molecular oxygen (such as air or oxygen gas) as an oxidizing agent have been proposed:
(1) a method in which a water-soluble salt of molybdenum, tungsten and/or uranium is used as a catalyst, and a primary amine is subjected to photo-oxidization in the presence of water and molecular oxygen by using a mercury lamp (see German Patent No. 1021358);
(2) a method in which a primary amine is subjected to oxidation under superatmospheric pressure in the presence of a catalyst, a tertiary alcohol and molecular oxygen, and preferably, further in the presence of gaseous ammonia, wherein the catalyst is tungstic acid, phosphotungstic acid, molybdic acid, selenic acid, selenious acid and the like (see Examined Japanese Patent Application Publication No. Sho 47-25324);
(3) a method in which a primary amine is subjected to oxidation in the gaseous phase in the presence of a catalyst and molecular oxygen, wherein the catalyst is a silica gel, alumina catalyst or a solid catalyst comprising alumina and tungsten oxide (see U.S. Pat. Nos. 4,337,358, 4,504,681, 4,560,797 and 4,624,939); and
(4) a method in which a primary amine is subjected to oxidation in the liquid phase under superatmospheric pressure in the presence of a homogeneous or heterogeneous catalyst comprising a metal belonging to Group 4 of the Periodic Table, and molecular oxygen (see European Patent No. 395046).
However, these methods (1) to (4) have the following defects. The above-mentioned method (1) requires the use of light and, thus, this method requires not only a large amount of electricity, but also a cumbersome operation for the maintenance of a light source, such as a mercury lamp. Further, in methods (1) and (2), a homogeneous catalyst is generally employed and, thus, these methods have a problem in that the separation of the catalyst from the reaction system after the reaction becomes difficult.
The reaction in method (3) is a heterogeneous reaction which uses a solid catalyst. Since the heterogeneous reaction is performed in the gaseous phase under relatively stringent conditions, namely at a reaction temperature of 120 to 250° C., it is considered that the catalyst is easily deactivated. The present inventors have studied this method and found that, when the reaction is performed in the gaseous phase at a reaction temperature of 160° C. or more, the catalyst is easily deactivated by the accumulation of the by-products on the surface of the catalyst, wherein the by-products include tar-like by-products (which are considered to be derived from the produced oxime) and other high boiling point organic compounds.
Further, it is known that an oxidation reaction of a cyclic aliphatic primary amine is an exothermic reaction, and an oxime which is a desired product is thermally unstable. Therefore, for performing an oxidation of a cyclic aliphatic primary amine on a commercial scale, a reaction in the liquid phase is advantageous over a reaction in the gaseous phase, because the heat of reaction can be removed more easily from the liquid phase. Further, there is a demand for the development of a method in which the reaction can be performed not only under mild temperature conditions (i.e., at low temperatures) which suppress the gradual decomposition of the produced oxime, but also by using a heterogeneous catalyst which can be easily separated from the reaction mixture.
With respect to the above-mentioned method (4), the above-mentioned European Patent No. 395046 discloses a method in which a primary amine is oxidized in the liquid phase under superatmospheric pressure in the presence of a heterogeneous catalyst comprising a metal belonging to Group 4 of the Periodic Table. In this patent document, reactions using titanium oxide and the like as the heterogeneous catalyst are exemplified. However, the selectivity for the produced oxime is as low as approximately 30 to 50%.
As apparent from the above, the conventional methods had the following problems. With respect to the methods for producing an oxime by using a peroxide as an oxidizing agent, the use of the peroxide is dangerous and cumbersome operations are necessary. Further, the above-mentioned methods (1) to (4) which use molecular oxygen as an oxidizing agent had problems, such that a cumbersome operation is needed for the separation of the catalyst, that the catalyst is easily deactivated, and that the selectivity for oxime is low. Therefore, it has been desired to develop a method which has advantages that the deactivation of the catalyst can be suppressed, that an oxime can be produced with high selectivity even when the reaction is performed at relatively low temperatures, and that a solid catalyst, which can be easily separated from the reaction mixture, is used.